1. Field of the Invention
The present invention relates to a micro coaxial cable, and more particularly to a micro coaxial cable in which foaming cells are formed uniformly in an insulation layer not to show any local difference of dielectric constant in the insulation layer, thereby giving excellent transmission characteristics.
2. Description of the Related Art
A coaxial cable includes a center conductor for transmitting signals, and an outer conductor coaxially formed on the center conductor, and many coaxial cables have been developed in various sizes and various kinds. The coaxial cable is generally used for transmitting signals to CATV or underground antennas. The development of coaxial cables has been mainly directed to a structural design between the center and outer conductors for reducing loss of energy, improvement of dielectric characteristics, and endowment of various schemes to an outside of the outer conductor.
In particular, along with the progress of high-technology information-oriented society, there is recently more demand for high-speed transmission rate of information communication devices and test/examination devices of semiconductor elements applied to the information communication devices.
A conventional coaxial cable includes an inner conductor 11, an outer conductor (or, a metal shield layer) 17, a polymer insulation layer (or, a dielectric layer) 13 formed between the inner conductor 11 and the outer conductor 17, and a protective coating layer 19 formed on the outer circumference of the outer conductor 17, as shown in FIG. 1. A transmission rate of the coaxial cable is determined by a dielectric constant of the insulation layer. That is, the transmission rate increases as the dielectric constant is lowered, and the dielectric constant is lowered as the degree of foaming of the insulation layer increases.
Thus, in the prior art, in order to get a high transmission rate and a low loss, a fluoric resin having a low dielectric constant is used for the insulation layer, a mixture ratio of general resin is controlled, or a supporter for supporting a gap between the center conductor and the outer conductor is used such that an air layer having a lowest dielectric constant surrounds the center conductor. In particular, the recent study is mainly focused on the technique for lowering a dielectric constant by foaming polymer material.
Meanwhile, portable multimedia devices and medical instruments such as an endoscope are recently extremely small-sized, so a micro coaxial cable having a diameter of 1 mm or less for driving the above devices is under development. The micro coaxial cable includes an inner conductor, an insulation layer, an outer conductor and a protective coating layer, which is basically similar to the traditional coaxial cable. The micro coaxial cable uses a high frequency in GHz range on occasions, and in this case ‘Skin Effect’ occurs due to the high frequency transmission, so a dielectric constant of a polymer insulation layer surrounding the micro coaxial cable is an important factor in the transmission characteristics of the micro coaxial cable. Meanwhile, in the polymer insulation layer in which a region having a foaming cell is crossed with a region having no foaming cell, the dielectric constant may vary locally due to the ‘Skin Effect’, which may cause a fatal ill effect to the transmission characteristics. Accordingly, ‘uniformity of foaming’ in the polymer insulation layer becomes a very important factor. Conventional general or large-sized coaxial cables have a common diameter of 5 to 42 mm so an insulation layer has a sufficient thickness, thereby keeping a uniform outer diameter during the foaming process and also capable of realizing uniform foaming sizes. However, the micro coaxial cable has a whole diameter of 1 mm or less, so foams may grow abnormally or an outer diameter becomes unbalanced. In particularly, since an insulation layer has a thickness of merely about 0.05 mm, irregularity of the insulating thickness causes local differences of dielectric constants, thereby resulting in deterioration of transmission characteristics. There are persistently many endeavors for solving such disadvantages in the relevant fields, and the present invention is designed under this circumstance.